This invention relates to a method and an apparatus for the control of wastewater flow to a wastewater treatment plant (WWTP) in order to reduce the cost of electricity during times of high electrical charges. The invention discloses the use of the sewage collection and conveyance networks as temporary storage units in order to either reduce peak flow rates or to shift the time period when the peak flow reaches the WWTP.
Municipal sewage commonly flows thru a network of variable sized sewer and interceptor pipes from residential homes and industries to wastewater treatment facilities. The wastewater treatment process at such facilities is energy intensive, with an approximate annual US electrical bill in the range of $500 million to $1 billion dollars. Because of the natural diurnal variation in wastewater flow, wastewater flow rates at a WWTP tend to increase during the day to coincide with the time period of higher electric rates. However, sewer networks typically have redundant and excess storage capacity to meet infrequent extreme flows. Sewers are generally designed for a 50-yr life and also include excess capacity expected to be required during the design life of a sewer. This excess capacity can be used to store and then deliver sewage flow to the treatment facilities so the wastewater can be treated in order to take advantage of lower electric rates.
Electricity is used by a WWTP for a variety of uses such as compressing air for biological treatment processes, pumping liquids, and process sludge. The electrical usage is related to the flow of sewage into the plant and increases as the flow increases. In general, wastewater flow is high during the day and low at night while the cost of electricity is higher during the day than at night. Thus the cost of electricity varies with the time of day. Another cost factor is the peak demand of electricity (maxmium short-term power drawn during the billing period) by the WWTP.
The prior art describes a number of ways that WWTPs have utilized to lower electrical costs. These methods include, providing on-site electricity generating capcicity, replacing existing electrical motors with higher efficiency motors, shifting electrical use, where possible, to periods of lower demand and replacing existing processes with higher efficiency processes. Also, replacing coarse bubble aeration with fine bubble aeration will usually result in a significant electrical savings as will the implementation of a dissolved oxygen control strategy and instrumentation. None of these methods to lower WWTP electrical costs teach flow shifting within the existing sewer system as a way to reduce electrical costs or to provide flow equalization.
In order to minimize the electricity expense, the sewage flow into the plant needs to be controllable so that high flows occur when electricity rates are low and low flows occur when electricity rates are high, so that the new flow profile favors low peak demands. In general, electric utility rate structures vary among companies and between states and WWTPs and typically are subject to complex rate structures classified under the heading industrial use. Rates are generally affected by supply and demand with incentives to load shift and reduce peak demand during high use (daytime) hours.
Wastewater treatment plants are typically designed and built to treat a fluctuating flow stream, one that varies widely throughout the day and possibly the season in both quantity and strength. These variations in loading require process units and equipment be large enough to meet reasonable daily peak loadings, periodic seasonal peak loadings such as rain events, and the projected demand imposed by future growth. By controlling the flow to the process at a more even rate, loadings are more consistent, and biological and energy demands are more stable. Through the dampening effects of equalization, only the treatment units and equipment needed to meet the equalized loadings are required to operate. One method known to accomplish equalization is the use of equalization basins which have a volume generally less than 35% of the WWTP capacity. They can be located on site or upstream of the WWTP and arranged as separate in-line or off-line tanks. The result is an overall improvement in WWTP efficiencies, more consistent removal rates, reduced electrical peak-demand charges, and possibly decreased power consumption. Additional benefits include the dissipation of shock loads that most WWTPs experience and the extension of the operating capacity within the existing facility because the initial design capacity was oversized to allow for peak demands. The beneficial effects of equation for biological wastewater treatment processes are described in the U.S. Pat. No. 5,853,589 issued to G. Desjardins on Dec. 29, 1998.
It is known from the prior art that sewage overflows from combined sewer interceptors results in continued degradation of receiving water quality. The prior art describes the construction of deep tunnels for the capture and storage of combined sewage and storm water runoff. These tunnels collect storm water and sewage during a storm so that it is not bypassed to the receiving water. Following the storm, the collected storm water and sewage in the tunnels is then pumped to the treatment plant. Off-line basins or tanks are most widely used. In cases where stormwater and wastewater are combined in sewers, flow rates are regulated by system design features so that wastewater treatment plant capacities are not exceeded. Historically, diversion or regulator structures were required to divert flows in excess of the treatment plant capacity to the surface receiving water via the combined sewer overflow outlets. Examples of flow control devices used for diversion of storm flow include U.S. Pat. No. 3,974,655 issued to R. Halpern on Apr. 14, 1975, U.S. Pat. No. 4,167,358 issued to J. Besha on Sep. 11, 1979 and U.S. Pat. No. 5,321,601 issued to D. Riedel on Jun. 14, 1994. Therefore, the use of existing sewer capacity to eliminate or minimize diversions from combined sewers is known. Flow control devices are also available for preventing back-flow in sewer systems. Examples of these include U.S. Pat. No. 5,406,972 issued to G. Coscarella on Apr. 18, 1995, U.S. Pat. No. 4,503,881 issued to F. Vecchio on Mar. 12, 1985 and U.S. Pat. No. 4,494,345 issued to B. Peterson on Jan. 22, 1985. None of these flow control devices are described for use to temporarily reduce diurnal on-peak flow rates to a WWTP on a regular basis for the express purpose of lowering electrical costs.
It is known in the prior art to utilize flow control devices in networked systems to optimize sewer system capacity and thereby reduce the amount of wastewater that is bypassed to surface waters during extreme flow events. None of these networked systems are described as being useful to temporarily reduce diurnal on-peak flow rates to a WWTP on a regular basis to lower electrical costs for the WWTP.